1. Field of the Invention
The present invention relates to sucker rods and has particular reference to a method for making sucker rods.
2. Description of the Prior Art
Thread-interconnected sucker rods serve for transmitting reciprocating motion from the pumping unit situated at the wellhead, to the oil-well sucker-rod pump located on the bottom section of the sucker rod string along which oil is extracted from the producer well.
Sucker rods that have up till now been made solid in compliance with Standard APJ Spec 11B, are in fact long-length cylinder-shaped bodies provided with threaded heads at their ends.
The head-to-body transition assumes a long-sweep curve, while the generant of the transition is in effect a circular arc tangent to the body generant and having its center located off the rod. Such a form of the head-to-body transition is required for reducing stress concentration effective in the transition zone both during the rod operation and for holding the weight of a suspended rod string during the round-trip operation with the aid of a special device, i.e., elevator having a surface on which a contact with the rod transition portion occurs, said surface being shaped so as to suit the shape of such transition portion. It is due to such a form of the head-to-body transition that the rod surface in contact with the elevator is protected against local mutilation and premature wear which might subsequently prove to be a regent of corrosion-fatigue cracks and eventually a complete failure.
When in operation sucker rods undergo a complex cyclic loading while imparting reciprocating motion to the pump plunger and are attacked by corrosive fluids contained in the oil extracted and are subjected to bending when operating in directionally slant drilled or inclined wells. Threaded joints of the sucker rods are also subject to wear during their making-and-braking.
Such a complex cluster of effects act on the sucker rods that they are exposed to high stresses causative of failures in the threaded joints and within the zone of the body-to-head transition.
It is due to the fact that, when in operation, sucker rods are positioned one above another and have to operate continuously within a prolonged period of time, that especially strict requirements are imposed upon their operating reliability. High reliability of sucker rods is the more so necessary as failure of one of the rods results in damage to all other rods of the string, which involves shutdown of the well and expensive repair work.
Thus, reliable operation of sucker rods is the principal criterion of their construction.
Taking account of mass production of sucker rods and the considerable expenses for their manufacture, and, high consumption of expensive alloyed steel used for their production, there has been proposed that sucker rods be of a welded construction rather than a solid one.
Known in the art is a method for making sucker rods composed of a rod body and end heads (cf. British Patent No. 2,104,936A, Cl. E 21 B 17/00 published on Mar. 16, 1983) which consists in their pressure-welding, followed by turning of the weld joint zone with formation of heat-affected areas both in the rod body and end heads.
When carrying said method into effect use is made of the heads whose configuration corresponds to that of solid sucker-rod heads as per Standard APJ Spec 11B. Pressure welding, in particular, friction welding is carried out on surfaces having equal cross-sectional area which is equivalent to that of the rod.
Such a method for making sucker rods cannot assure their operating reliability for a number of reasons.
The welded joint is established across the minimum rod cross-section and is located at the least favorable rod portion, that is, at the place of the body-to-head transition, where considerable effect is exerted by the constructional stress concentrator associated with the rod cross-section variation.
Inevitable inaccuracy of head-to-body welding, which is due to imperfect positioning of components to be weld-joined in a welding machine and dimensional errors of the blanks being welded, results in shoulders and other imperfections unavoidably left after flash elimination and affecting adversely the corrosion-fatigue strength characteristics of the sucker rods.
The initial structure of metal is modified in the heat-affected zone as a result of high-temperature heating during the welding procedure, which also tells adversely on the load-bearing capacity of a welded joint and makes it practically impossible to efficiently use prehardened rolled stock for manufacture of sucker rods. This is of special importance since heat-and especially heat-and-strain hardened rolled stock from carbon or low-alloy steels features high strength and plasticity characteristics, as well as increased fatigue limit when exposed to the effect of a corrosive medium, which renders it suitable for making welded sucker rods therefrom rather than from expensive alloyed rolled stock.